1. Field of the Invention
This invention relates to a vertical power semiconductor device having a guard ring layer in its termination section and a method for manufacturing the same.
2. Background Art
The low breakdown voltage MOSFET (metal oxide semiconductor field effect transistor) is widely used in notebook personal computers, AC adaptors for LCD (liquid crystal display), and switching power supplies for servers, and is subjected to demands for reduced on-resistance and increased operating speed.
In a conventional low breakdown voltage MOSFET, the channel resistance accounts for a large proportion of the on-resistance. Hence, reduction of on-resistance has been achieved by reducing the channel resistance by downscaling, for example. After the channel resistance was reduced to a certain level, reduction of the drift resistance has been required.
One of the methods for reducing the drift resistance is to use the super junction structure (hereinafter also referred to as “SJ structure”), which has a proven performance in high breakdown voltage MOSFET (see, e.g., JP-A2006-278826(Kokai)). The SJ structure is a structure in which, for example, P-type pillar layers are periodically arrayed in an N-type semiconductor layer. In the SJ structure, the depletion layer can be extended horizontally from the interface between the P-type pillar layer and the N-type semiconductor layer. In contrast, without pillar layers, the depletion layer extends only vertically from the interface between the N-type semiconductor layer and the P-type base layer. Hence, upon application of the same drain voltage, the SJ structure can extend the depletion layer more widely and achieve a higher breakdown voltage than a structure free from pillar layers. Thus, even for an increased impurity concentration in the N-type semiconductor layer serving as a drift layer, the required breakdown voltage can be maintained, and the drift resistance can be reduced. Also for a low breakdown voltage MOSFET, it has been reported that the semi-SJ structure, in which the P-type pillar layers are formed halfway through the N-type semiconductor layer, can reduce the on-resistance and improve the tradeoff between the breakdown voltage and the on-resistance.
Another method proposed for reducing the drift resistance is to provide a buried electrode below the trench gate and apply to this buried electrode a potential equal to that of the source electrode (see, e.g., JP-A2002-083963(Kokai)). This structure can also achieve the same effect as the SJ structure so that the impurity concentration in the drift layer can be increased to reduce the drift resistance.
However, the SJ structure and the buried electrode structure described above have a problem in that the breakdown voltage of the termination section is decreased with the increase of impurity concentration in the drift layer.